1. Field of the Invention
This invention relates to heat exchangers that are used as reactors to catalytically dehydrogenate endothermic hydrocarbon fuels in propulsion systems with particular application to gas turbine engine combustors. The present invention provides apparatus and methods for avoiding thermally induced fuel deposits on the catalytic surfaces of these reactor heat exchangers.
2. Description of Related Art
It is well known to design aircraft propulsion systems to use endothermic hydrocarbon fuels. Endothermic fuels hold the potential for significantly increasing the performance of gas turbine and other engines as a result of their heat sink potential. One particularly important endothermic hydrocarbon fuel is Methylcyclohexane (MCH) which can be catalytically converted to toluene and hydrogen with a theoretical absorption of 1959 Btu/lbm. This example includes both the sensible and chemical absorption of heat and assumes a 99% conversion and that the fuel was heated from 70.degree. F. to 1340.degree. F.
The practical implementation of converting MCH to toluene and hydrogen requires the use of a heterogeneous catalyst. Reforming catalysts conventionally used in the conversion of petroleum naphtha feedstocks to high octane gasoline are also used in the present invention. These catalysts essentially dehydrogenate cyclo paraffinic reactants to produce the desired result. Such catalysts include, but are not limited to Pt-Al2O3, Pt-Re-Al2O3, and Pt-Ir-Al2O3. Over the range of temperatures and pressures in which these reactions typically occur (400.degree.-900.degree. K. and 1 to 150 atmospheres) competing reactions take place which include cracking, re-hydrogenation, and coke formation. The formation of coke in particular, is a problem as it tends to strongly adsorb on the surface of the catalyst, fouling active sites and reducing catalyst surface area and pore passages, thereby reducing overall catalytic activity.
It has been reported in the petroleum industry that the inclusion of hydrogen (H2) in the reactant feed suppresses the formation of coke, see "Catalytic Reforming of Naphtha in Petroleum Refineries" by M. Dean Edgar, published in Applied Industrial Catalysis, Volume 1, 1983; "Coke Tolerance of Catalytic Reforming Catalysts" by In-Sik Nam, John W. Eldridge, and J. R. Kittrell, published in Ind. Eng. Prod. Res. Dev., 1985; and "Reaction Kinetics of Methylcyclohexane Dehydrogenation over a Sulfided Pt+Re/Al2O3 Reforming Catalyst" by Michael A. Pacheco and Eugene E. Petersen, published in Journal Of Catalysis, 1985.